Resin bonded article



Patented Aug. 9, 1938.

. I nnsm nommn sn'rrctn Rupert S. Daniels,'Eaat Orange, N. J., assignor to Bakelite Corporation, New York, N. Y., a corporation of Delaware NoDrawing. Continuation oi application Serial No. 321,773, November 24, 1928. This applica H]! March'30, 1934, Serial No. 718,202. In Cam I ada October 22, 1929 12 Claims. (01. si zaoi' 'This invention-relates to bonded granular or fibrous articles and more particularly to abrasive implements such as grinding wheels, and to methods for their preparation.

5 Abrasive implements such as grinding wheels and the like play a very important part in the manufacturing industries. As an instance among the various operations in which such implements are utilized there may be noted that of finish-grinding whereby articles are brought to their flnal form and dimension. In this operation it is customarily essential that the ground articles have surfaces free from variations and conforming with exactness to the desired .dimensions, such as roller bearings which require a true circular cross section that is accurate within ex tremely small limits. Hard-bonded wheels such as the vitrified bonded wheels, are not satisfactory as they tend to cause chatter marks on the work, particularly if there is the slightest bond being those of elasticity and loss of strength under the action of undue heat generated in a grinding operation. Finish-grinding operations also-involve the requirement of not marring or injuring the work through undue pressure or .30 other action which may result in over-heating,

and'with a soft bond this difliculty is overcome by the weakening action of the generated heat on the bond. For these and other reasons softbond wheels have a place in the industries which cannot be filledby hard-bond wheels.

To further distinguish hard and soft bonds, it may be observed that hard-bond wheels apparently depend primarily on the expansion ofv the abrasive grains caused by heating to crack the 40 grains or the bond and thus release worn-out contact portions, for as the grains become worn and rounded they present increased contact surfaces and thus generate and absorb more heat; in soft-bond wheels it-is the loss of strength of the bond due to heating and the consequent giving away of the bond that releases the worn abrasive grains. In other words, the expansion of the abrasive grains is a material factor in the continued abrasive action of hard-bond wheels, but it plays no part in the action of soft-bond wheels.

As heretofore prepared, however, abrading implements of the soft-bond type have depended upon shellac for contributing the properties deemed essential in finish-grindingand similar different batches do not possess the same characteristics. It is therefore not only a diflicult matter toreproduce wheels or other implements having the same characteristic of softness, but 5 the wheel properties being dependent on the particular batch of shellac selected, are not subject to the degree of control desired in practice.

According to the present invention there is provided bonded abrasive'implements and the 10 like in which the binder is a "soft bond but prepared from a synthetic resinous product free from the objections associated with the use of shellac, in that its properties can be accurately controlled and duplicated at pleasure to the end '15 that abrasive implements with definite fixed properties of elasticity, break-down under the action of heat, etc., can be reproduced atwill. In addition the binder of the present invention providesmarked advantages over shellac, since 20 the properties found essential for soft-bonded implements are not dependent upon the properties inherent in the substance itself as is the case with shellac, but can be varied through very wide ranges by the preparation and treatment 25 of the synthetic product.

Synthetic resins as binCers have heretofore found extensive application in the production of grinding wheels and the like. For instance phenol-formaldehyde resins that are hardenable 30 under the action of heat to the insoluble, infusible form are largely used in the preparation of cut-oif wheels and other types where there is a demand for a bond that will not soften under the high temperatures generated, or in other words, '35

resin and by treatment of the abrasive imple- 46 ment made therefrom be utilized in the manufacture of soft-bond implements. By properly controlling the preparation of these resins it has been found that they can be used as bonding agents for abrasive grains in the preparation of 50 grinding wheels or other molded articles without loss of shape or segregation of resinousbinder from the abrasive grains during a heating or baking to impart bonding strength. The invention involves the further discovery that the bonding 55 .strengthimpartedtoan-alkydresincanbeao controlled as to regulate its softening as demanded in practice. The wheels so prepared show marked uniformity of resin distribution throughout them, and they are further characterized by a degree of toughness or transverse strength under the normal grinding conditions for which they are intended such that thin discs are practical without the addition of reinforcement. By the controlled softening of the synthetic resinous bond, grinding wheels so prepared are accordingly well fitted for finish-grinding and similar operations requiring a soft-bonded wheel to avoid injury as well as to insure exact final dimensions of manufactured articles.

As compared with shellac-bonded wheels, the alkyd resin wheels of the softening type as described herein show on the average about twice the strength at room temperature even though their softening points are about as low. The superior strength and toughness of the softening alkyd resin wheels permit much higher operating speeds, and the softening property in conjunction with the other properties make them peculiarly useful as coping wheels for the working of granite and the like. Stone-cutting and similar operations require sharp edges, for as soon as these become rounded the cutting action is correspondingly reduced. In the use of a coping wheel streams of water are directed against the sides thus keeping them cool, but the cutting face of the wheel becomes heated. The bond being thus softened on the face more than on the edges permits a more rapid separation of the abrasive grains from the face than from the edges. Consequently the edges are maintained sharp and rounding is overcome.

While not limited to any particular alkyd resin or process of preparing it, it is found preferable to use, particularly for the bonding of wheels with a relatively high softening point, a resinous product obtained by boiling a mixture of about two parts of phthalic anhydride with one part of glycerol (these proportions correspond to 1% mols of phthalic anhydride to 1 mol. of glycerol) until the water of reaction is eliminated, and continuing the heating at a temperature ranging from 140 to 200 C., or even as high as 230 or 235 C., to the point of incipient gelation (as more fully described in a patent to Schmidt No. 1,739,- 771 granted December 17, 1929). The preferred condition of the initial reaction product so obtained is such that it is fusible but sufilciently solid when cold so that it can be subdivided or ground and the ground particles do not readily coalesce upon standing in the cold; to insure this condition, it may be found necessary to pour the product while hotand liquid into a mold or vessel.

and bake it with or without pressure in air or inert gas at a temperature varying from 140 to about 200 C. for a period that may vary from one hour and up to four hours or more. The product so made has the characteristic of being heat-reactive in the sense that it can be advanced or transformed by heating to a less fusible state or condition.

The initial reaction product is finely subdivided when cold, preferably to a size that will pass through a 100 mesh screen or finer. In view of its tendency to soften, however, the grinding to a fine mesh; particularly by a mill other than a hammer mill, may involve some difficulty. It is therefore preferred, particularly in the manufacture of grinding wheels or similar articles that include an abrasive or granular filler, to add the a,ias,sas

abrasive or filler to a roughly ground or broken resin, whereupon the mixture is submitted to the action of a ball mill. By this procedure, the abrasive itself is utilized for obtaining a very fine subdivision of the resin and at the same time it is dispersed. with substantial uniformity throughout the resin. All the abrasive may be added prior tothe ball mill operation, or a part may be mixed with the resin and the remainder added after or during the ball mill operation. An advantage residing in the finely subdivided or powder form is that the resin is found to be maintained in suspension when mixed with abrasive grains, thereby contributing to the formation of the bonding film and the uniform structure oi the finished article.

In the manufacture of grinding wheels, the finely divided alkyd resin is' normally mixed with about seven times its weight of abrasive grains until a uniform distribution is secured; these proportions are subject to wide variation. The prepared mixture can then be charged as such into a mold hot enough to cause welding of the resin particles under the pressure applied or the mixture can be previously heated to secure the same result. The mixture can be subjected to the usual pressures of 1,500 to 2,000 pounds per square inch, or in some cases the pressure may run as high as 6,000 pounds to the square inch. When heat is applied to weld the resin particles, it is then usually desirable to cool the mold to insure freedom from sticking.

While an article is in the mold, heat may be applied to harden or advance the resin bonding agent to an extent sufficient to give the needed grinding strength without further treatment. Preferably, however, the articles are removed from the molds and hardened by being placed in an'oven and baked. For example, the initial baking temperature of the molded article can be as high as 85 C. or even higher, without causing resin segregation or loss in shape of the article, and after a period of about 16 hours the temperature can be raised gradually to about 135 C. or even as high as 200 C., or higher, the baking being continued for a length of time depending on the degree of resistance to softening desired in the finished article and the thickness of the article. As a specific instance, to produce wheels of about one-half inch thick that will not soften under temperatures generated in normal finishgrinding practice, the baking or hardening is continued at about a temperature of 135 C. for 40 to 45 hours; higher temperatures correspondingly shorten this period. As the effect of the baking is to harden or polymerize the resin content of the articles, the division of the baking operation into a low temperature stage followed by a longer and higher temperature stage is adopted to insure evaporation and removal of any volatile solvent, prior to a partial hardening of an outer resinous layer, or to avoid hardening of an outer layer in advance of an interior portion to such an extent that cracks will be caused to appear.

In the abrasive art there are two well known forms of molding practiced, cold-molding and hot-molding. Cold-molding (which is generally more desirable commercially particularly when implements of appreciable thickness are to be prepared) may require a liquid or solvent for temporarily bonding the mixture, unless sufficiently heavy pressure is applied to cause welding or coalescing of the resin particles. But for hot-molding a liquid may also be added with advantage to assist in securing a dispersion of the menses resin particles among the abrasive grains in the preparation of the resin-abrasive mixture prior-- to its molding.

When a solvent is included to assist in cold moldin the underlying purpose in view is. to wet the grains so that the powdered resin will adhere to them. For instance, solvents suitable for this purpose include cellosolve (monoethyl ether of ethylene glycol), acetone, etc. and better results are obtained if a solvent is diluted with a non-solvent liquid; such as kerosene or creosote oil, to enhance dispersion. For hot-molding compositions solvents can beusedhut a resin non-solvent alone. such as kerosene, has been foundparticularly useful. The addition 'of" a non-solvent, such as kerosene, to a hot-molding tially or'entirely soluble, can be used both for warm and cold molding with good results. The liquid used is preferably added in a manner so as to wet the alkyd resin and grit particles but not to dissolve the resin to any materialextent; for example, it can be added gradually or at one time to the resin and grit mixture during a ballmill operation, or the grit can be coated with the liquid prior to the addition of the flnelydivided resin. v

Solubility or-partial solubility in solvents generally is characteristic of the alkyd resin only when it is in a very fusible form, that is, when the resin has not been advanced to any material extent by heat-hardening, unless heat is applied to the resin in the presence of the solvent at a sufflciently high temperature to retard'the resin structure and cause solution. But in order to insure that segregation of the alkyd resin from the v cause powdered alkyd resin in such an advanced stage to adhere thereto can best be obtained by incorporating with a solvent for the resin a proportion of an alkyd resin thus forming a varnish, for example, a 20 per cent solution of alkyd resin in carbitol (diethylene glycol monoethyl 'ether). As another example of a varnish suitable for use as a wetting agent may be mentioned an alkyd resin containing minor percentage of solvent, for example, such a resin having 20 per cent water content. The varnish or solution (hereinafter referred to as the wetting agent) so prepared is first mixed with the abrasive grains in order to coat them and the powdered advanced alkyd resin is then added. Up to 30 per cent or more of the total resin content can be incorporated in the varnish, and the varnish before applying can be heated to advance the resin therein. Non-solvents can be included in the varnish or solution as indicated above.

The foregoing procedure in connection with alkyd resins when a solvent'is to be included differs markedly from the known practice as applied to binders prepared from heat-reactive phenolaldehyde resins. As set out in the Martin'Patent No. 1,626,246, for instance, the abrasivegrains after coating with a solvent for the resin,such as furfural, are mixed with the powdered phenolaldehyde resin while it is yet in soluble form; this is possible with a phenol-aldehyde resin because the transformation to a more advanced form .where it becomes infusible occurs with relative rapidity so there is no danger of segregation or separation of the resin during theheat-hardenresin is advanced to the point where it loses solubility, it is at the same time so far advanced with regard to infusibility that a knitting or welding of the resin particles does not take place to the extent desirable to give adequate strength to the finished article. Accordingly such articles are not as satisfactory as those prepared with a powdered heat-reactive phenol-aldehyde resin in the readily soluble condition.

A surprising and unexpected result following from the advancement of a powdered alkyd resin as stated, therefore, is that the strength of the finished article is promoted rather than weakened provided a varnish or resin solution is included as a wetting agent. As an illustration a varnish was preparedby dissolving 20 grams of an initial reaction product as described above in grams of carbitol; about 350 grams of #12 silicon carbide grains were mixed with 10 grams of this solution, and after stirring until the grains were uniformly coated, about 50 grams of a powdered alkyd resin (advanced by baking as described above to a substantially insoluble form where it could be ground without coalescing upon standing) were added and the mixture stirred until uniform. This mixture was charged into a cold mold and pressed into shape under apressure of 2,000 pounds per square inch. The molded articles were then baked overnight at, to C. whereupon the temperature was raised to. 150: C. and the baking continued for about 50 hours. The baked articles were found to be uniform without exhibiting any segregation of resin.

To give an indication of strength and the effect of temperature, test bars were prepared from a 1 :7 resin and abrasive mixture including a varnish and advanced powdered resin in accordance with the foregoing description; the bars were df'xlflx in dimension. When tested cold a transverse strength of about 6,200 pounds per square inch was obtained, and when tested at 170 C. the strength was reduced to about pounds per square inch; the modulus of elasticity was almost 2.0x 10, and the impactstrength about 2.2 foot pounds per square inch. When placed in an oven on supports 5 inches apart and a 100 gram weight suspended at the'middle, a softening action was noted at 100 C., bending began at about -140 0., and breakage occurred at around (3.; in the progressive softening, the binder did not fuse but became cheesy or crumbly and lost its strength. In this respect the alkyd resin binder differs in a surprising manner from a heathardening resin binder prepared from a phenol and formaldehyde; with a hardened phenol resin binder following the process of the Martin patent including 14 per cent of furfural based on the resin, similar. test bars showed no softening and were not affected when a 300 gram weight was suspended at the middle and the bars heated overnight at l80-l90 0.; they showed a transing period. If the powdered phenol-aldehyde verse strength of about 4,800, a modulus of elasticity of about 2.5)(' and impact strength of about 1.7 foot pounds per square inch. The alkyd resin binder also differs markedly in the nature of the softening that takes place from the shellac binder, for the latter has a tendency toward fusion or becoming gummy upon heating.

In the preparation of the varnish or solution, various solvents can be used as for example hexalin, acetone, ethyl lactate, diethyl phthalate, diethyl oxalate, dipropyi ketone, glycerol, glycol, water, or mixtures of these or mixtures with nonsolvents, such as toluene, naphthalene, halogenated or chlorinated cyclic hydrocarbons (as naphthalene, diphenyl, anthracene, phenanthrene), kerosene, creosote oil; etc., specific examples of such mixtures being cellosolve (monoethyl ether of ethylene glycol) .with toluene, hex-' alin with naphthalene, etc. These solutions are preferably made by pouring the hot liquid resin reaction mass directly into the solvent whereby the resin reaction is arrested and solution obtained. The proportions of resin and solvent can vary within wide limits, though a solution that is liquid at room temperature is preferred as the coating medium; however, more viscous or even solid solutions that fuse upon heating can be used by providing heat at the necessary temperature to cause coating of the abrasive grains.

The proportion of the total resin supplied in a wetting agent need not be large so long as there is a sufficient amount present to exert a preferential action of causing the solid resin particles to adhere to the grains instead of to each other. For example the resin incorporated in' the solvent may be but 1 per cent of the total resin content or it may rise as high as per cent or more. Nor need the dissolved resin and the powdered solid resin be of the same composition; for instance a resin prepared from succinic acid in place of phthalic anhydride with glycerol or other polyhydric alcohol can be used in the varnish or solution and a phthalic anhydride-glycerol resin for the powdered solid resin.

High pressures are not required when a solvent is present. The composition in such a case can be pressed into the mold by a trowel, rolling cylinder, or other suitable means. When so compacted, the article can be removed, for with the usual proportions of filler and resin, the molded piece has sufilcient stiifness at normal room temperatures to permit handling.

In place of the allwd resin alone as a resinous constituent of the binder, it has been found that mixtures of alkyd resins and phenol resins can be used. By means of such mixtures it is possible to further regulate the strength, toughness, softening point, etc., of the resin binders. Mixtures of alkyd resins and phenol resins can be prepared for example, by the use of mutual resin solvents. As an illustration a resin mixture was prepared by homogenizing 80 parts of a fusible alkyd resin with 20 parts of a heat-hardening phenol-aldehyde resin as by means of a common solvent. The resin mixture was reacted to the point where it solidified upon cooling and could be ground. 350 grams of abrasive grains were coated with 2% cc. of an alkyd resin varnish as prepared above, and to the coated grains were added 50 grams of the powdered resin mixture. Test bars of a 20 per cent phenol resin and 80 per cent alkyd resin showed a strength of about 5,000 pounds per square inch, a modulus of elasticity of 2.1 10 and impact strength of about 1.8 foot pounds per square inch. In general, it is found that these resin mixtures do not exhibit the strength or toughness of the alkyd resin alone but they do provide bonds of greater density and harder wheels. In this way, therefore, the increased density of the wheel which is generally desired in polishing operations can be had without objectionable increase in the strength of the bond.

The porosity of the article can be regulated by the size of the grit, or the use of mixed grains of difl'erent sizes, and to some extent by the proportions of grit to resin. For example, the percentage of resin may be as high as 20 per cent by weight, or thereabouts, and as low as 5 per cent by weight, or less, and give a satisfactory bonded product, the article with the higher resin content having a greater surface density. Density is also increased by hot pressing or molding, and a very dense article can be obtained when a solvent, such as cellosolve, is included in the mixture for hotmolding. On the other hand, the inclusion of a non-solvent, such as kerosene, gives an open structure of uniform character, particularly when pressed in a cold mold and thereafter baked, though the transverse strength is then somewhat reduced. In general, hot-molded articles have higher transverse strength than those which are cold molded.

By alkyd resin is to be understood any of the synthetic resins prepared from a polyhydric alcohol and a dibasic or polybasic acid and with or without the addition of a monobasic acid or other modifier. While the preceding description refers specifically to a glycerol-phthalic anhydride resin, other resins resulting from glycerol or other polyalkyi alcohols and other polybasic acids can be substituted to thereby obtain diiferent temperature gradients. For example glycol and carbohydrates may be substituted for glycerol, and succinic, maleic, tartaric and other polybasic acids for phthalic anhydride; as monobasic acids there may be included oleio, palmitic, stearic, etc. (see patents to Callahan No. 1,108,329 and Arsem No. 1,098,776). Changes in properties can also be secured by varying the proportions of the resin reactants as for instance by decreasing the proportion of phthalic anhydride which lowers the softening point. It is also possible to vary the wheel properties by including catalytic agents; for example a metal oxide as zinc oxide, iron oxide etc. can be mixed with the abrasive grains so that when the resin binder is added and the mixture molded the resin sets up or hardens with increased rapidity during the baking operation. By these various means-extent of baking, proportions, modifying ingredients, etc.-wheels can be produced which soften at practically any temperature varying from 100 C. or less and up to 200 C. or higher.

Furthermore, though the above description is specifically directed to grinding wheels using an abrasive as a filler, the invention is applicable to the manufacture of other articles and with'the use of any other filler, either powdered, granular or other structural forms, both natural and artiflcial, and of a size which gain by being bonded together and generally designated as granules; the invention therefore is not to be interpreted as restricted to a particular resin, filler or article of manufacture, but to be given a scope commensurate with the appended claims.

This application is a continuation of application Serial No. 321,773 filed November 24, 1928.

- I claim 1. Method of preparing an abrasive article of the soft-bond type from granules having as a bond an alkyd resin which comprises mixing the granules with the alkyd resin in powder form, molding the mixture into an article, and baking the molded article,-when the proportion of resin to granules is about one to seven and the thickness oi the article cold-molded under pressures of about 1,500 to 2,000 poundsper square inch is about oneehalf inch, at about 85 C. for about 16 hours and continuing at about 135 to 150 C. for about to 50 hours, the heating period being in inverse ratio to the temperature and in direct ratio to the thickness, resin content and porosity of the article.

2. Method of preparing an abrasive article of r the soft-bond type from granules having as a bond an alkyd resin which comprises coating the granules with a wetting agent, mixing the coated granules with the alkyd resin in powder form, molding the mixture into an article, and baking the molded article, when the proportion of resin to granules is about one to seven and the thickness of the article cold-molded under pressures of about 1,500 to 2,000 pounds per square inch is about. one-half inch, at about 85 C. for about 16 hours and continuing at about 135 to 150 C. for about 40 to 50 hours, the heating period being in inverse ratio to the temperature and in direct ratio to the thickness, resin content and porosity ofthe article. 3. Method of preparing an abrasive article of the soft-bond type from granules having as a bond an alkyd resin which comprises advancing an alkyd resin to a state of incipient gelation, grinding the resin so produced into a powder passing through a screen of 100 mesh, coating the granules with a wetting agent, mixing the coated granules with the alkyd resin in powder form.

molding the mixture into an article, and baking the molded article, when the proportion of resin to granules is about one to seven and the thickness of the article cold-molded under pressures of about 1,500 to 2,000 pounds per square inch is about one-half inch, at about C. for about 16 hours and continuing at about to C. for about 40 to 50 hours, the heating period being in inverse ratio to the. temperature and'in direct ratio to the thickness, resinacontent and porosity of the article. J

'4. Method of preparing an abrasive article 0 the soft-bond type from granules having as a bond an alkyd resin which comprises mixing the granules with the alkyd resin in powder form, molding the mixture into an article, and baking the molded article to advance the resin to a state of bonding strength characterized by a softening that becomes crumbly without substantial fusion under grinding pressure 'at temperatures ranging approximately from 100 to 200 C.

'5. Method of preparing an abrasive article of the soft-bond type from granules having as a bond an alkyd resin which comprises coating the granules with a wetting agent, mixing the coated granules with the alkyd resin in powder form,

molding the mixture into an article and baking the molded article to advance the resin to a state of bonding strength characterized by a softening that becomes crumbly without substantial fusion under grinding pressure at temperatures ranging approximately from 100 to 200 C.

6. Method of preparing an abrasive article 01 the soft-bond type from: granules having as a bond an alkyd resin which comprises advancing an alkyd resin to a state of incipient gelation,

grinding the resin so produced into a powder passing through a screen of 100 mesh, coating the granules with a wetting agent, mixing the coated granules with the alkyd resin in powder form, molding the mixture into an article, and baking the molded article to advance the resin to a state of bonding strength characterized by a softening that becomes crumbly without substantial fusion under grinding pressure at temperatures ranging approximately from 100 to 200 C.

7. In a method of preparing a bonded article of the soft-bond type from a granular filler and a finely divided alkyd resin, the step which comprises including monoethyl ether of ethylene glycol in the mixture of filler and resin.

8. In a method of preparing a bonded article of the soft-bond type from a granular filler and a finely divided alkyd resin, the step which comprises including diethylene glycol monoethyl ether in the mixture of filler and resin.

9. In a method of preparing a bonded article I of the soft-bond type comprising an abrasivev filler: and a bonding agent in powdered solid form, the step which comprises subjecting the mixture of filler and agent to the action of a ball mill to secure subdivision of the agent.

' 10. Bonded article of the soft-bond type comprising granules secured by a binder comprising an alkyd resin included in the form of a powder passing through a screen of 100 mesh and baked to a state characterized normally by strength and toughness but softening by crumbling without substantial fusion under grinding pressures within a temperature range approximating 100 to 200 c. I

ll. Bonded article of the soft-bond type com? prising granules secured by a binder comprising an alkyd resin baked to a state characterized normally by strength and toughness but softening by crumbling without substantial fusion under grinding pressures within a temperature range approximating 100 to 200 C; 12. Bonded article of the. soft-bond type comprising granules coated by a resin solution and secured by a binder comprising an alkyd resin included in the form of a powder passing through acterized normally by strength and toughness but fusion under grinding pressures within a temperature range approximatinglOO" to 200 C.

IIRIUPIEIR'I S. DANIELS.

' a screen of 100 mesh and baked to a state char- I softening by crumbling without substantial Q0 

